It is typical in the plastics industry that reinforcements and fillers are used to improve the properties of said materials. Commonly, this is done to achieve improvement in physical properties such as tensile strength, flexural modulus and impact strength. Common fillers are talc, calcium carbonate and other minerals. By far the most common fibrous reinforcement is glass. Glass fibers impart high strength, dimensional stability and heat stability. There are commercial requirements that glass fiber and other mineral fillers cannot fully meet.
For example, glass fibers increase the density and cost of the material and abrade processing equipment. In addition, glass fibers are manmade, non-renewable materials with a considerable environmental impact.
Cellulosic materials have been evaluated as reinforcements in the past. In particular, wood fibers from a variety of sources have been extensively studied, together with the effects and improvements that common processing aids would be expected to impart to such systems. Typically, wood fibers have a lower density (1.3 g/cc) than glass fibers (2.6 g/cc) or other minerals (e.g. wollastonite, 2.9 g/cc). From an ecological point of view, wood fibers have a reduced environmental impact.
Chemically, wood fibers (and flours) are not pure cellulose, they contain at least two other major components, hemi-cellulose and lignin—with different varieties and different species of planting containing different ratios of these constituents. When the wood fibers are processed with thermoplastic resins at temperatures that exceed 200° C., the fibers exhibit severe discoloration and thermal degradation. There is also significant off-gassing and objectionable odors, principally due to impurities and the lignin that modify manufacturing process and create a worker safety concern. It has also been shown that processing temperatures above 200° C. reduce the physical property improvements delivered by the fibers (Klason, et al., Inter. J. Polymeric Mater., Volume 10, p 175 (1984)). Furthermore, when the moisture present during the processing of the cellulosic materials is not controlled and/or eliminated, performance of the resultant composite is compromised. Management of moisture in the materials and the molded parts has proved costly and difficult, most commonly being achieved through fiber pre-processing or encapsulation of the fiber in the thermoplastic.
Wood fibers are hydrophilic materials that are poorly wet by molten hydrophobic thermoplastic polymers resulting in poor flow characteristics, difficult processability, and premature fiber pull-out, all of which leads to molded products with poor appearance and inferior mechanical properties. Property improvement has been addressed through the use of “compatibilizing” materials that adhere to both the polar fibers and the non-polar polymer resin. Most common has been the use of a maleic anhydride grafted polypropylene copolymer. See, for example, U.S. Pat. No. 5,948,524 and “Wood Flour Filled Polypropylene Composites: Interfacial Adhesion and Micromechanical Deformations”, L. Danyadi et al., Polym. Eng. & Sci. 47(8), pp 1246-1255 (August 2007). Typically, wood flour-polypropylene composite (WPC) pellets are manufactured via compounding extrusion equipment followed by thermoplastic post-forming processes such as injection molding or profile extrusion. The engineering of desirable processing and physical properties into a WPC is further complicated by the addition of other substances such as reinforcing glass fiber filler, non-cellulosic particulate fillers, colorants and mold release compounds. As many physical property requirements are associated with homogeneous distribution of fillers and the matrix wetting of those fillers by the olefinic matrix, a chemical coupling package ideally is compatible not only with WPC itself, but also glass fiber filled and non-cellulosic particulate filled forms thereof.
None of the wood polymer composite materials that have been developed to date afford a completely satisfactory set of performance characteristics and as such are deficient for end-use applications in one or more of, e.g., poor physical properties, water resistance, undesirable odor, poor processability and/or excessive cost. These shortcomings stem from drawbacks in a forming process having poor melt flow characteristics due to inadequate adhesion between the dispersed wood fibers and thermoplastic polymer matrix at high wood fiber loading levels.
It is evident that to make a practical wood plastic composite end product, such as an injection molded spindle, toys, automotive parts, etc., the wood plastic composite material must be dried prior to processing. Additionally, wood plastic compounded pellets can be blended with other virgin or compounded polymers in pellet form. These additional pellets may have ingredients for reinforcement such as glass and or mineral, to produce various types of molded products that are tailored to particular end-use applications.
Thus, there exists a need for a thermoplastic composition with improved cellulosic inclusion-thermoplastic interactions relative to conventional composites so as to create superior performance molded articles.